Alternative exon splicing is a widely utilized mechanism for the control of gene expression and the production of protein isozymes in eucaryotes. The Drosophila melanogaster muscle myosin heavy chain (Mhc) gene encodes a remarkable diversity of isoforms that are differentially expressed in specialized muscles through precisely regulated alternative exon splicing. Molecular and genetic experiments are proposed to investigate mechanisms that control the muscle-specific alternative exon splicing of this Mhc gene and to test specific hypotheses of splicing and regulation involving alternative exon donor choice mechanisms. Candidate splice junction and intron sequences responsible for regulation of flight muscle-specific alternative exon splicing have been identified by comparative analysis of Drosophila virilis and Drosophila melanogaster Mhc sequences. Mutagenesis and transgenic studies of minigenes expressing alternative exon domains will define the specific functions of these putative regulatory sequences in splicing regulation. Trans-acting genes that regulate flight muscle- specific alternative exon splicing choice will be identified by genetic studies of dominant and recessive mutations that disrupt alternative exon splicing and by biochemical studies to identify molecular components that interact with embryonic regulatory sequences and splice donors to control muscle-specific alternative exon choices. The proposed studies are expected to define basic mechanisms of alternative exon splicing regulation during cellular differentiation. Such understanding likely will provide insights into the genetic basis of human diseases involving aberrant gene expression and may lead to the development of gene therapy strategies to control aberrant gene expression by targeted disruption of specific splicing components and splicing reactions.